This invention relates to a molded product of synthetic resin. More particularly, it relates to a synthetic resin molded unit with a desired surface hardness and a method of making the same.
Synthetic resin molded products, for example thermoplastic molded parts, are widely used in many fields of industry for a wide variety of applications such as for structural components, articles used in household, business, daily life, and the like, since they may be easily produced. These thermoplastic molded parts are excellent in durability. However, when they are employed for uses such as transparent windows for meters or clocks, dust covers for record players or tape recorders, lenses of flash lights, transparent lenses or covers for automobile head and tail lights, a variety of mirrors, and the like, they are subject to abrasion on their exterior surface due to external elements such as dust which cause a deterioration in their appearance. More important, in cases where they are used for windows of meters, the scales and indicators may become unreadable after prolonged use. Similarly, in cases where they are employed for lenses, this abrasion causes a decrease of diffusion in the transmission of light, thus rendering such products useless. These disadvantages and deficiencies are evident in the case of conventional plastic molded parts, and until now, they have been unavoidable without additional manufacturing procedures. If these disadvantages and deficiencies are resolved, the durability life of such plastic molded parts can be extended, resulting in a great improvement in the value thereof, as well as an increase in the possibilities for expanded applications for molded plastics.
As an approach to eliminate such abrasion on the surfaces of plastic molded parts, the surfaces have been coated with a hard coating to improve their surface-hardness. Hard-coatings such as epoxy resin derivatives, acryl resin derivatives, amino resin derivatives, polysiloxane derivatives, and the like have been known for some time. Some of them have even been used in industry on a limited scale. Each of these hard coatings is, in fact, dissolved into a solvent and coated on an injection-molded part by spraying the solution or by immersing the molded part in the solution, so that a hard-coating layer, having a thickness ranging from a few microns to less than one hundred microns, is formed on the molded part. The resulting hard-coating layer on the molded part is then cured by means of heat or ultraviolet irradiation. Although this conventional coating process may improve surface-hardness, it has been undesirable for the industrial production of plastic molded parts requiring surface-hardness because it requires 3 steps, a molding step, a coating step and a curing step. Moreover, this process results in the following additional drawbacks:
(1) Numerous substandard products are created during the curing process
1.1 An injection molded-part invariably has molecular orientation and/or residual stress. The heat treatment used to cure the hard-coating layer on the surface of the molded part causes shrinkage deformation in the molded part due to changes in this molecular orientation and/or residual stress, leading to a change of the overall dimensions of the product. Cracks or crazings result and sub-standard products are produced.
1.2 The heat treatment used to cure the hard-coating layer on the surface of the molded part causes changes in dimensions of the molded part resulting in products, so that they do not conform to a required size or shape.
1.3 Since the plastic material used in the manufacture of the molded parts has a tendency to be charged with static electricity, as long as the conventional three-step technique is applied, it is very difficult to keep the molded part free from dust during the period between the molding process and the coating process. Accordingly, the coating on the surface of the molded part becomes uneven, thereby making it impossible to attain an even surface-hardness. Due to the above production problems resulting from the conventional 3-step technique, it has been found that the number of sub-standard units are about 10% when manufactured on an industrial scale.
(2) Lack of uniformity of surface-hardness
2.1 It is impossible to achieve a hard-coating layer having a uniform thickness on the surface of molded parts by means of the spray coating or the immersion-coating technique. The resulting surface layer lacks uniformity of its surface-thickness resulting in a lack of uniformity in surface-hardness. This is especially true where the molded part has caves or convexities on its surface, the thickness of the hard-coating layer becomes irregular so that its external appearance becomes extremely poor.
(3) Complications in the coating process
3.1 In cases where a hard-coating is only necessary on one surface of a molded part, the surface which is not to be coated must be masked while the other surface is being coated. When this non-coated surface has an irregular form or structure, the masking means used must be made to match this irregular form.
(4) High cost
4.1 Conveyance of the molded parts between the various steps is extremely expensive and labor consuming because of their packaging and bulkiness.
4.2 In the event that the place of production of the molded parts is different from the place in which the hard-coating treatment is conducted, unnecessary labor and expense is required in order to prevent the molded part from being damaged or subjected to dust during transport and/or storage before the hard-coating is applied.
4.3 In the event that the molded parts become contaminated by dust before the hard-coating is applied, unnecessary labor and expense is required to clean them before the hard-coating can be applied.
4.4 Coating equipment is very expensive since it must be equipped with a large scale baking-furnace, replaceable by one another depending on a shape of the molded part to be coated and necessary for each place of production of the molded parts.
4.5 In order to prevent dust from contaminating the molded part due to static electricity and to attain the aims of making a hard and even surface on the molded part, large scale equipment is required for purification of air and/or for maintaining a dust-free atmosphere during the coating process.
The abovementioned problems have been a major drawback to the use of surface-hardness treatment of the synthetic resin products such as plastic molded parts. Due to these disadvantages and deficiencies of the prior art, surface-hardness treatment of a variety of resin products has never conducted on an industrial scale.